The difference is whether you just receive or transmit also (respectively, C vs. B).

I think that it very useful and a great feature -- expecially with a plotter that you're using. By having a Class B, you can (or are) being seen by other traffic. With just a C, you're only getting part of the benefit.

We find it useful even in the Chesapeake or in coastal waters with barge and fishing traffic.

Essential, probably not.